Group-III nitride semiconductors have a direct transition-type band gap at energy levels corresponding with a range from visible light to the ultraviolet light region and also exhibit excellent light emission efficiency, and have therefore been widely commercialized as semiconductor light-emitting elements such as light-emitting diodes (LED) and laser diodes (LD) for use in all manner of applications. Further, even when used within electronic devices, group-III nitride semiconductors have the potential to yield superior properties to those obtainable using conventional group III-V compound semiconductors.
Generally, group-III nitride semiconductors are produced by a metal-organic chemical vapor deposition (MOCVD) method, using trimethyl gallium, trimethyl aluminum and ammonia as raw materials. MOCVD is a method in which vapors of the raw materials are incorporated within a carrier gas and transported to a substrate surface, and the raw materials are then decomposed by reaction with the heated substrate, causing crystal growth on the substrate surface.
Examples of materials that are known to be usable as the substrate include insulating substrates such as sapphire, and conductive substrates such as silicon carbide, silicon, zinc oxide and gallium arsenide, but a substrate that enables perfect lattice matching with group-III nitride semiconductors is yet to be developed, and currently, blue LED elements prepared by enforced growth of a group-III nitride semiconductor layer on a sapphire substrate in which the lattice constant mismatch is 10% or greater are being used in practical applications.
Conventional blue LED elements have a double hetero structure in which, basically, an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer composed of a group-III nitride semiconductor are laminated in sequence on a sapphire substrate. As mentioned above, sapphire has insulating properties, meaning an electrode cannot be drawn from the substrate side of the element, and therefore the element is produced with a so-called face-up structure or flip-chip structure in which a p-type electrode and an n-type electrode are provided on the same surface of the group-III nitride semiconductor layer.
However, conventional elements having face-up structure or flip-chip structure that use sapphire as a substrate have numerous problems. Firstly, because the p-type electrode and the n-type electrode are aligned horizontally, the electric current flows in a horizontal direction, and as a result, the current density tends to increase in localized areas, causing the chip to heat up. Secondly, because the sapphire that is used as the substrate is extremely hard and does not exhibit cleavability, high level technology is required to generate the chips. Thirdly, the thermal conductivity of sapphire is comparatively low, and therefore the heat generated in the group-III nitride semiconductor layer cannot be dissipated efficiently.
In order to address the above types of problems, WO05/029572 discloses a method of producing a light-emitting diode having an upper-lower electrode structure in which a group-III nitride semiconductor layer is formed on top of a plating layer. In other words, WO05/029572 discloses a method for producing a light-emitting diode having an upper-lower electrode structure, the method including: sequentially laminating an n-type semiconductor layer, a light-emitting layer and a p-type semiconductor layer on top of a sapphire substrate to form a group-III nitride semiconductor layer, forming a p-type ohmic electrode on one surface of the p-type semiconductor layer, forming a seed layer on top of the p-type ohmic electrode, subsequently forming a photoresist in a lattice shape on top of the seed layer, thereafter forming a plating layer that covers the seed layer and the photoresist, subsequently removing the sapphire substrate and then forming an n-type ohmic electrode on the n-type semiconductor layer, and finally, removing the photoresist and performing dicing.